In the art of window assemblies having main window frames and sash frames formed of extrusions of a plastics material or polyvinylchloride, commonly referred to as vinyl, it has been found desirable in some installations, especially in residential and commercial buildings near the ocean and Gulf coast, to provide for keeping the window assembly in tact during a high windload, for example, during a hurricane or tornado. For example, U.S. Pat. No. 6,826,871, which issued to the assignee of the present invention, discloses a double hung or single hung window unit constructed of vinyl extrusions and incorporating hook members on the sill and bottom sash member. The hook members engage in response to a high negative windload to limit deflection of the sash frame and thereby avoid permanent deformation and/or separation of the sash frame. It is also desirable to limit deflection of a movable sash frame or separation of the sash frame from the surrounding main frame when the window glazing and glass panel unit are subjected to a high negative or positive windload after impact by an object sufficient to break the glass panel unit, such as during a hurricane or tornado.